Japanese Unexamined Patent Application Publication No. 2008-143293 discloses control for detecting the running position of a vehicle and calculating the value of current to be supplied to a steering actuator in accordance with a target steering angle of a steering wheel, the current steering angle, and the current vehicle speed such that the running path of the vehicle becomes identical to a target path. In this control, the value of current supplied to the steering actuator is calculated in accordance with an estimated coefficient of friction with a road surface.
In Japanese Unexamined Patent Application Publication No. 2008-143293, an appropriate control amount is calculated in accordance with a coefficient of friction with a road surface, thereby improving the convergence of a steering angle to a target steering angle, and, regardless of the coefficient of friction with the road surface, promptly making the running path of the vehicle identical to the target path.
By the way, if the running path becomes deviated from the target path in a situation where the vehicle speed is high, the roadway is sharply curved, or the coefficient of friction with the road surface is lowered due to rain or snow, it is difficult to correct the deviation. Even if the vehicle is controlled with a great control amount in such a case where the deviation is great, the vehicle may not be able to sufficiently follow the target path. Furthermore, the occurrence of sudden acceleration/deceleration or steering is not desirable.